The resilient growth of air travel significantly impacts the environment through emissions of greenhouse gases and other pollutants. These chemical species affect climate change, air quality, human health, wildlife and agriculture. Aviation currently accounts for around 3–5% of anthropogenic climate change, a share likely to rise due to increasing passenger demand and the challenge of implementing effective mitigation solutions. Besides carbon dioxide (CO₂), aircraft produce short-lived climate forcers like nitrogen oxides (NOₓ), water vapor (H₂O), contrails and aerosols, notably soot and sulfate (SO₄). Many of these lead to highly uncertain warming and cooling effects. The magnitude of these effects strongly depends on how pollutants are transported and chemically transformed throughout the atmosphere – processes that are influenced by flight altitude and seasonal conditions. This dissertation aims to advance our understanding of aviation’s climate effects by investigating the transport patterns of pollutants like NOₓ and SO₄ through the development and application of a novel Lagrangian tagging method.

Aviation is an important contributor to anthropogenic climate change. As indirect greenhouse gases, nitrogen oxides (NOx = NO + NO2) add to the Effective Radiative Forcing (ERF) induced by aircraft emissions. This work addresses a better understanding of emission mitigation by investigating dependencies between engine parameters and emitted nitrogen oxides as a part of the campaign VOLCAN. In-flight near field measurements are conducted using the DLR research aircraft Falcon, chasing an Airbus A321neo equipped with CFM LEAP-1A engines featuring lean combustion through staged fuel injection. Nitrogen oxide concentrations are measured applying the well-established chemiluminescence method. Emission indices are quantified for different fuels including Jet A-1, Sustainable Aviation Fuel (SAF) and two SAF blends with different levels of aromatics. Four combustor inlet temperature settings are tested and staged (lean) combustion is compared to unstaged (rich) combustion. Emissions are measured for two different but technically identical engines, deviating in terms of exhaust gas temperature margin. As expected, results indicate no significant differences in emitted nitrogen oxides for the investigated fuels. Measurements confirm that nitrogen oxide emission indices increase exponentially with combustor inlet temperature. Due to the forced nature of the analyzed rich burn mode, a reduction in nitrogen oxide emissions through lean combustion cannot be confirmed. Based on presented data, a relationship between engine degradation and nitrogen oxides is likely. Near field observations agree with well-established far field measurements and lead to lower uncertainties. Preliminary results of the ECO-Demonstrator campaign are in line with VOLCAN measurements. The performed research is a highly valuable contribution to extremely rare empirical in-flight emission data. Established nitrogen oxide dependencies support technical decision making to reduce aviation’s climate impact.

The adoption of hydrogen as an alternative fuel in aircraft has the potential to reduce the climate effect of aviation significantly. However, hydrogen leakage during production, storage, or use can offset these benefits by altering atmospheric chemistry and composition, particularly through interactions with methane, ozone, and stratospheric water vapour. This study employs surrogate models based on recurrent and convolutional neural networks to simulate the climate effects of hydrogen leaks, achieving rapid projections 30,000 times faster than conventional climate models, with an error margin of less than 5%. This efficiency enables the quantification of uncertainties related to hydrogen leakage rates and atmospheric chemistry through Monte Carlo simulations, allowing for an assessment of their contributions to radiative forcing under various Shared Socioeconomic Pathway (SSP) climate scenarios. By 2100, the radiative forcing from aviation-induced hydrogen leaks is projected to reach 43.9 ± 21.2 mW m-2 (±1σ) under the stringent climate change mitigation projection SSP1-2.6, accounting for 35% of aviation’s total radiative forcing. Under the more conservative scenario SSP3-7.0, higher methane levels reduce the oxidative capacity of the atmosphere, lowering the projection to 17.7 ± 6.9 mW m-2 (±1σ) which corresponds to approximately 5% of the aviation’s total radiative forcing for this scenario. These findings demonstrate that hydrogen leaks have the potential to substantially contribute to aviation’s total radiative forcing, with the magnitude of their impact heavily influenced by background climate conditions. It will be important to minimise hydrogen leakage to fully harness the climate benefits of transitioning to hydrogen as a fuel for aircraft.

Reducing anthropogenic climate change is a significant challenge requiring a global response to prevent tipping points in the climate system, such as the disintegration of ice sheets, and thawing of permafrost, among others. The rapidly growing air transport sector, which carried 4.5 billion passengers in 2019, is projected to emit nearly 2 Gt CO2 by 2050—about 2.6 times the emissions in 2021. Decarbonising aviation is challenging due to its reliance on fossil fuels, and while technological, operational, and regulatory measures have reduced fuel consumption, they are insufficient to mitigate aviation’s overall climate impact. The non-CO2 effects are significant, accounting for about two-thirds of aviation’s warming impact in terms of Effective Radiative Forcing (ERF). These effects include contrails, contrail-induced cirrus clouds, nitrogen oxides (NOx ), and water vapour emissions, collectively contributing to approximately 4% of anthropogenic forcing since the pre-industrial era. Given their spatio-temporal variability, climate-optimised flight planning canmitigate these impacts by avoiding sensitive regions, but this faces several challenges. These include the inherent chaos of weather, low scientific understanding of non-CO2 effects, and the large computational expense of calculating sensitive regions using climate change functions (CCFs).

To address these issues, this thesis first analyses algorithmic climate change functions (aCCFs), a simple surrogate model obtained by regressing the CCFs against local atmospheric variables. The aCCFs are computationally inexpensive to run since they only use few meteorological inputs to estimate climate impact, enabling real-time flight trajectory optimisation on arbitrary days. However, aCCFs are applicable only in parts of the Northern Hemisphere and require thorough verification before implementation. The focus is narrowed down on local aviation NOx effects on climate change, which largely causes warming via short-term increase in tropospheric ozone (O3) and is characterised by large variability. This necessitates a detailed investigation of NOx-O3 effects in isolation and its mitigation, which is a previously unexplored area. After verifying the O3 aCCFs through complex climate-chemistry model simulations, it is concluded that while it enables a reasonable first estimate, there are a few discrepancies.

TheO3 aCCFs are replaced by using a more comprehensive dataset comprising global NOx-O3 impacts, identifying additional physical variables that influence this impact, and using this information to train stochastic surrogates based on homoscedastic and heteroscedastic Gaussian processes. These models provide mean and uncertainty estimates for the climate impact of NOx on O3, for the first time. The heteroscedastic model more accurately reproduces the data distribution and its ease of use in predicting the climate impact of individual flights is demonstrated. Defined as probabilistic aCCFs (paCCFs), these models demonstrate superior accuracy over aCCFs, provide valuable insights for aviation’s non-CO2 effects, and offer broader implications for climateoptimised flight planning. The thesis concludes with limitations and recommendations to furthermitigate aviation’s environmental impact.

At speeds roughly between five and ten thousand km/h, hypersonic aircraft offer the promise of an extremely fast means of transport. Growing concerns about climate warming, however, direct attention to sustainability. This thesis focuses on atmospheric composition and radiation changes by considering a range of individual hypersonic aircraft designs on trajectory and route network level.

State-of-the-art Earth system models are used for simulations, and results calculated with the EMAC model are subsequently compared with simulations performed elsewhere with the LMDZ-INCA model. The comparison to a third model, i.e. WACCM, with a very similar – but independent – model setup allows even further clarification. For model validation satellite measurements (ozone, water vapor) and aircraft measurements (ozone, water vapor, temperature) are taken into account.

After the introduction in the first chapter, the second chapter is a general description of the Earth system including anthropogenic perturbations, in particular perturbations from subsonic, supersonic and hypersonic aircraft emissions followed by a detailed explanation of methods and the EMAC model setup in the third chapter. A new research finding in the context of middle atmospheric chemistry is the increased methane and nitric acid oxidation following hypersonic emissions. This effect results in a (photo-)chemical net production of water vapor and eventually increases water vapor perturbations further, which is described in detail in chapter 4. In chapter 5 an analysis of atmospheric dynamics and transport of emitted trace gases in the middle atmosphere underlines the importance of the Brewer-Dobson circulation and shows the impact of polar stratospheric clouds on water vapor perturbations during polar winter. The evaluation of multiple hypersonic aircraft designed for different cruise altitudes shows that their climate impact increases with cruise altitude and can be approximately 10-20 times as much as a conventional aircraft (chapter 6). Emissions at different hypersonic cruise altitude and latitude regions show that the climate impact can vary more with latitude of emission than with altitude of emission (chapter 7). With rf_of_hypersonic_trajectories() a software was developed to estimate the climate impact of aircraft design and flight trajectory/network options in seconds based on robust results from Earth system modelling. Using the software it is shown that a cruise altitude optimization loop can reduce the overall climate impact of a state-of-the-art aircraft design (chapter 8).

There are two methodological highlights to mention in the context of the EMAC model. The first is a new MESSy submodel H2OEMIS, which was created as part of this thesis. H2OEMIS is an interface to include water vapor emissions in EMAC model simulations, which was not possible before. This submodel will generally be of interest for future evaluations of e.g. any vehicles emitting water vapor and the impact of volcanic eruptions with EMAC. The secondmethodological highlight is the application of a novel speed-up technique during simulation runs, which reduces the simulated years by twothirds. To conclude the summary, the four following points are important to take away. This thesis brought

• A new research finding on middle atmospheric chemistry: The identification of a chemical feedback that enhances the water vapor perturbation lifetime albeit an increasing chemical water vapor destruction
• A robust estimate of the climate impact of hypersonic aircraft for both specific aircraft designs and general atmospheric and radiative sensitivities showing a large altitude and latitude dependence
• An easily accessible tool for researchers and companies to estimate the climate impact of new hypersonic aircraft designs with low cost and low time
• An estimate how the development of hypersonic aircraft would contribute to a road map to a climate optimal aircraft industry compared to conventional aircraft

Cirrus clouds play a crucial role in the Earth’s energy budget. They reflect incoming sunlight and absorb and re-emit terrestrial infrarred radiation. The magnitude of these components depends on the cirrus micro physical properties (e.g., ice crystal number and effective diameter), which can result in either net warming or cooling effects. This thesis investigates the differences in those properties of high- and mid-latitude cirrus, as well as their interactions with atmospheric aerosol and aviation-induced contrail cirrus. To explore these differences, cirrus clouds were measured with the German research aircraft HALO (High Altitude and LOng Range) during the CIRRUS-HL (CIRRUS at High Latitudes) campaign in June and July 2021. A total of 24 flights and 35 hours of in situ measurements of cirrus particle data were collected with the Cloud Droplet Probe (CDP), the Cloud Imaging Probe Grayscale (CIPG) and the Precipitation Imaging Probe (PIP). A comprehensive intercomparison among the instruments, along with a detailed uncertainty analysis, was performed, resulting in an accurate and quality-controlled data set. The results of these measurements show that high-latitude cirrus, compared to mid latitude cirrus, have lower median ice number concentration (0.001 and 0.0086 cm−3), higher median effective diameter (210 and 165 μm), and lower median extinction coefficient (0.042 and 0.072 m−1, respectively). In addition, high relative humidity over ice is observed, particularly at high latitudes, with median values around 125%. The influence of the formation region on cirrus properties was assessed by combining measurements with weather model data and backward trajectories starting at the flight tracks. The results show that a large part of the high-latitude cirrus were formed at mid-latitudes, leading to different properties compared to cirrus formed directly at high latitudes. Simulations from an aerosol-chemistry-climate model were combined with the backward trajectories and a strong contribution of heterogeneous nucleation was identified in the measured cirrus. Thus, the low concentrations of ice nucleating particles at high latitudes (from the model) combined with high ice supersaturation levels might explain the lower ice number concentration and larger effective diameter of cirrus measurements at high latitudes compared to mid-latitudes. Aviation emissions have a large local impact on the cirrus microphysical properties through contrail formation and their evolution to contrail cirrus. The CIRRUS-HL data set shows a higher occurrence of contrail cirrus at mid-latitudes, and a potential impact on natural cloudiness by reducing supersaturation levels at cirrus altitudes. The effect of contrails from future propulsion technologies may depend on background aerosol concentrations. Comparisons of measurements and model data for total aerosol number concentrations show good agreement for the larger particle size mode (> 250nm), but likely an underestimation above 300 hPa in the Aitken mode (> 12nm). By combining observations with model data, this study contributes to enhancing the understanding of the variability in cirrus properties due to different formation mechanisms and aerosol influences, as well as the interaction of natural and contrail cirrus.

Icing conditions that contain supercooled large droplets (SLDs) represent a hazard for aviation. Due to their large inertia, SLDs impact behind the extent of aircraft ice protection systems where the developing ice accretion cannot be removed. At present, aircraft that are vulnerable to SLD icing need to avoid severe icing conditions in general, leading to increased operating costs. New American and European aviation regulations allow the passage through such conditions if the aircraft carries instruments that can detect the presence of SLDs. Upon detection, the pilots can exit the icing cloud before the situation becomes hazardous.
The mass of liquid water contained in an SLD is several orders of magnitude higher than the mass contained in a typical small cloud droplet. However, the number concentration of SLDs is much lower than that of small cloud droplets, consequently, it is challenging to detect SLDs with instruments. As of now, no instruments for the detection of SLD icing conditions are in use on commercial aircraft.
This thesis investigates a combination of two instruments for the detection of SLD icing conditions. The first instrument is the Nevzorov probe for the measurement of liquid and total water content and the second instrument is the Backscatter Cloud Probe with Polarization Detection (BCPD), a non-invasive laser backscatter probe that measures the size and shape of cloud particles between 2 and 42 µm. The Nevzorov probe used in this work carried a new, 12 mm diameter total water content sensor that was added to the instrument specifically for the measurement of SLD icing conditions. Both instruments, the Nevzorov probe and the BCPD, are first analyzed individually in icing wind tunnel tests. The findings from the tests show that the new 12 mm sensor of the Nevzorov probe captures SLDs effectively. No indication was found of SLD being incompletely evaporated due to splashing or of water being swept out of the cone. The collision efficiency of small droplets with the sensor is low and can be compensated with a correction derived in this work. Intensive atmospheric testing ensued the icing wind tunnel measurements. During measurements in Arctic clouds, it could be shown that the fraction of liquid and glaciated particles can be estimated from the BCPD. Measurements in the South of France were able to demonstrate that the detection and discrimination of SLD icing conditions is possible with the Nevzorov probe and the BCPD for SLD icing encounters that are sufficiently long and contain a high number of SLDs.
The results of this work allow future flight campaigns to use the 12 mm sensor of the Nevzorov probe and benefit from its capture efficiency and better sampling statistics. The comparisons of the Nevzorov probe to other instruments can help scientists choose suitable instrumentation for future icing wind tunnel and flight campaigns. Concerning the BCPD, a new method developed in this thesis to estimate the number of ice and water particles could, with small modifications, also be employed for other instruments that use polarization filters. For the detection and discrimination of SLD conditions, future work should focus on extending the sample area of the BCPD further outward from the aircraft skin to measure particle size distributions that are unaffected by the aircraft boundary layer. Furthermore, the false alarm rate of the system could be reduced by incorporating an instrument similar to the BCPD but with a larger size range and larger sample area for the direct detection of SLDs.

As global air traffic has continued to grow overthe past two decades, it has effectively led to a sharp increase in the emissions and corresponding global warming through the 'greenhouse gas effect'. Apart from CO₂ and H₂O, NO_x is the next major aviation-emitted species which significantly contributes to climatechange through its atmospheric chemistry leading to the formation of O₃,which in itself is another major greenhouse gas. 

Since global air traffic is a major source of tropospheric NO_x,this thesis analyzed the contribution of aviation to tropospheric mixing ratiosof NO_x and O₃. The analyses were performed on 2 majoraspects: (A) to identify and understand the seasonal & zonal patterns with respect to aviation's contribution to tropospheric mixing ratio of NO_x andO₃, and (B) to comprehend the differences between 2 methodologies(called “Perturbation” & “Tagging”) in estimating aviation's contributionto tropospheric mixing ratio of NO_x and O₃. Inaddition to NO_x and O₃, background availability ofradicals OH and HO₂ was also analyzed, since OH and HO₂ arevital to the NO_x-O₃ chemistry.

The global climate-chemistry model EMAC (European Centre forMedium-Range Weather Forecasts – Hamburg (ECHAM)/MESSy Atmospheric Chemistry) was used, whereby the simulations were performed in quasi-chemistry transport model (QCTM) mode. The analyses presented an interesting overview of aviation's contribution to tropospheric NO_x and O₃.

The aviation industry continues to contribute to anthropogenic climate change. Recent studies have shown that the total radiative forcing from aviation is around three times higher than that from CO₂ alone. To account for the full effect of aviation, climate metrics are needed, which equate the environmental impact of various emissions and effects. However, there is currently no consensus on which climate metric should be used in aviation policy. This thesis systematically analyses existing climate metrics by: 1) comparing the responses to simple emission profiles; 2) investigating the sensitivity to changes in high-level aircraft design variables; 3) using a Monte Carlo simulation to determine inherent biases in metric calculation methods; and 4) evaluating the ability of climate metrics to estimate CO₂-equivalent emissions. It is concluded that the Average Temperature Response (ATR) is the most appropriate climate metric for aviation climate policy. However, it is found that the time horizon remains a subjective choice that must be chosen carefully depending on the climate objective. The GWP*, a newly proposed climate metric, is concluded not to be suitable as a climate metric because of its high variability and secondary time horizon. Nevertheless, it is recommended to investigate the potential of using the GWP* as a Micro Climate Model to further climatic understanding.

Aviation activity results in around 5% of global climate impact, for which both the sources and the manner to improve through technology, operations, and policy need to be better understood. Therefore, a Scenario-based assessment through AirClim is done, assessing future efforts and their support for the Paris Agreement goals. The baseline showed 39% of climate impact resulting from CO2 emissions, with 61% from NOx-related effects and contrail-cirrus, resulting in 212.8 millikelvins of induced temperature change. Reduction of 75% climate impact in the year 2100 was possible but only through highly optimistic assumptions. Removing optimistic assumptions climate impact of 104.9 millikelvins is achievable. Comparison to relevant studies shows results to be comparatively high, besides uncertainty in component radiative forcing. More top-down and bottom-up studies must map clear boundaries to be imposed on e.g. airliners, manufacturers, and political bodies, to allow for structural climate impact reduction. Future developments are unlikely to meet Paris Agreement demands if not accompanied by a broader scope than CO2 emissions.

Contrail formation is one the largest warming contributions of aviation’s climate impact. Measures to mitigate contrail climate impact include the optimisation of flight trajectories to avoid the formation of warming contrails or to find air space where extra cooling contrails are formed. The latter option is only possible during daytime, due to the interaction of a contrail with the short-wave radiation from the Sun. Little research is done to evaluate the effect of the diurnal cycle on contrail climate impact mitigation. Moreover, models to predict this the contrail climate impact are still in the development face. This thesis aims to (1) find the daytime and nighttime contrail climate impact of eco-efficient flight trajectories, (2) to assess the prediction’s robustness and (3) to recommend a best practice for eco-efficient flying by using the difference between the daytime and nighttime contrail climate impact. To this end, simulations were carried out within the EMAC climate chemistry model, aided by the submodels AirTraf, CONTRAIL and ACCF. The robustness of the results were tested against a model parameter: the threshold time until sunrise (THsunrise). When a contrail is formed
during nighttime, THsunrise determines whether the contrail is formed sufficiently close to sunrise to be considered a daytime contrail. It is concluded that winter daytime mitigation of contrail climate impact is more eco-efficient than winter nighttime mitigation. Overall, daytime mitigation achieves a larger maximum reduction of contrail climate impact than nighttime mitigation. moreover, summer mitigation is more effective than winter mitigation and winter
mitigation has a larger reliance on the formation of extra cooling contrails. The thesis results are robust with respect to a varying THsunrise. However, overall results become biased to daytime results or nighttime results. Lastly, it is shown that differences between day and night contrail climate impact mitigation allow for the enhancement of mitigation results.

Aviation has a growing impact on the earth’s climate, due to its emissions causing an increase in the global near-surface temperature. In order to better understand the dynamics by which different aircraft types contribute to this global warming and how this can be mitigated, the impact of different aircraft categories is analysed and compared. These categories are constructed by dividing aircraft into groups with a similar number of seats. Two approaches are employed to assess the climate impact of these categories. First of all, a global aviation emission inventory is used together with a climate response model, to evaluate the temperature change caused by each individual category. It is found that in absolute terms the middle category with 152-201 seats, and the largest aircraft with over 302 seats will cause cause the largest temperature change by the year 2100, compared to 1940. At the same time, per amount of generated capacity in the form of available seat kilometres, the middle category shows the smallest climate impact of all, with the largest aircraft being the second worst. From historical positional data of aircraft, flight trajectories are identified, to which an aircraft performance model is applied. This leads to the conclusion that the optimal distance in terms of fuel use is »2500 km, with an increase in fuel burn for both longer and shorter distance flights. The NOx emission increases for increasing flight distance, due to higher thrust settings and a higher rated thrust of the engines used. Next to being able to fly longer distances, the three largest aircraft categories cruise at a higher Mach number, increasing fuel use and NOx emission. Additionally, by flying in a less busy airspace, such as above the Atlantic, the impact of contrails is larger for these aircraft, as there is a smaller atmospheric capacity to form contrails in these areas. Another reason that the largest aircraft perform worse is the higher level of comfort provided to the passengers. It is shown that increasing the seating density to maximum capacity leads to a reduction in fuel use and NOx emission per available seat kilometre especially for the largest aircraft, narrowing the gap with the smaller aircraft categories which already make use of a high density seating. Based on the outcome it is suggested to adjust the policies dealing with aviation’s climate impact, to reflect the differences between the impact of the different aircraft types. Additionally, a direct reduction in aviation’s climate impact can be achieved by making use of the most climate efficient aircraft type for a givenmission.

The climate impact of aviation is assessed as function of the emission altitude for two different aircraft types, the Boeing 787-800 and Boeing 777-300ER. The basis for this research is an assembly of 2,738 historical trajectories for which the fuel consumption and emissions were determined by using Piano-X aircraft performance data and atmospheric weather data from the European Centre for Medium-Range Weather Forecasts. The resulting emissions served as input for the climate response model AirClim which calculated the resulting climate response over time. To analyze the effect of changes in cruise altitude, the fuel consumption and corresponding emissions were recalculated for scenarios with relocated cruise altitude profiles ranging from an upward shift of 2000 ft to a downward shift of 18000 ft with respect to the original cruise altitude. By shifting cruise altitudes down, the total climate impact was found to be reduced for both aircraft types where the minimal climate impact is found for the lowest analyzed cruise altitude. The reduction in climate impact is mainly the result of the reduced short term forcings from contrail cirrus, ozone and the induced destruction of methane where their individual contribution to the total climate impact reduction was found to be dependent on aircraft type. Relocating cruise altitudes up was found to increase the climate impact for both aircraft types.

Aviation is a large contributor to anthropogenic climate change through its emissions, particularly through nitrogen oxides (NOx), which reacts to Ozone in the atmosphere. Due to the emission of NOx, the ozone budget of the atmosphere is disturbed which results in an increase in the Earth’s surface temperature. To mitigate this effect, the variability of NOx and ozone due to aviation must be properly understood, which can be assessed through measurements in the atmosphere. NOx from aviation has been successfully measured in fresh plumes of aircraft, however, research has pointed out that the variability of ozone in the atmosphere is rather high, making it doubtful whether the perturbation due to aviation is detectable except under specific meteorological conditions such as high-pressure systems. This thesis assesses whether there is a correlation between high-pressure systems and the concentration of NOx from aviation and its produced ozone in those systems. With the result of this assessment recommendations are formulated which aid in designing future measurement campaigns of ozone from aviation.

In order to reduce the climate impact induced by the aircraft industry, the variability between cost optimized and climate optimized flight trajectories was evaluated. This was done to establish how the climate impact reduction affects the operating costs and vice versa. To evaluate the climate impact of air traffic, a climate-chemistry model was used to simulate the earth its atmosphere and sub-models to study aircraft trajectories. This study considered 85 flights spread out over the European airspace, analyzed for six months, including realistic weather systems throughout the entire analysis spectrum. Cost and climate optimized trajectories were compared for flight altitude, horizontal flight paths, emissions and operating costs. The presence of a seasonal effect was examined which marks the difference between a winter and a summer season, and four different flight directions were considered that showed the effect of the wind speed and direction on the flight properties.

As global warming has become a prominent issue for mankind, scientists are working on every possible aspects to slow down its pace. Aviation emission contributes more and more to the anthropogenic climate impact. Contrails contribute to about 30% of the aviation induced RF. Among the options that are proposed to mitigate the formation of contrails, one of it is to reroute the aircraft in a way to avoid flying through the contrail formation regions (CFRs). To carry out this option, it is necessary to have a better understanding of the distribution of the CFRs as well as their seasonal variation and future trend. The CFRs analyzed in this thesis are simulated by EMAC model, the CFRs' characterisations studied include their zonal and meridional coverage length, stretch factor, direction in the vertical and horizontal planes as well as their seasonal variations and future trend.

Emissions of road traffic crucially influence Earth’s climate. The vehicle fleet emits not only carbon dioxide (CO2), but also nitrogen oxides (NOx), volatile organic compounds (VOC) and carbon monoxide (CO) which produce ozone (O3) and destroy methane (CH4) in the troposphere. As the demand of mobility is expected to further increase in future, a reduction of the climate effect from road traffic emissions is indispensable. Therefore, it is essential to assess the climate impact of emission changes caused by technological trends and mitigation strategies for road traffic. Several studies have already quantified the impact of road traffic emissions on climate. But climate simulations with complex chemistry climate models are still computational expensive hampering the assessment of many road traffic emission scenarios. Consequently, an efficient method for quantifying the climate impact and contribution of mitigation options is required. Within the scope of this thesis, a unique chemistry-climate response model called TransClim (Modelling the effect of surface Transportation on Climate) was developed. Using an efficient interpolation algorithm, it assesses the impact and the contribution of road traffic emission scenarios on O3 and CH4 concentration as well as their corresponding radiative forcings. Comparing the results delivered by TransClim with simulations of the complex global chemistry climate model EMAC reveals very low deviations (0.02 – 6 %). To determine not only the impact but also the contribution of road traffic emissions to O3, OH and CH4 in TransClim, a so-called tagging method is applied. It attributes the concentrations of trace gases to emission sources such as road traffic. This thesis presents an improved tagging method for the short-lived species OH and HO2 as well as a new method for CH4. Within the scope of this thesis, TransClim enabled to assess the climate effect of two scientific questions: first, the effect of three prospective mitigation options of German road traffic and second, two scenarios describing the cases that European vehicles use fuel blends containing a low and a high proportion of biofuels. Summing up, TransClim offers a new method to quickly assess the climate impact and the contribution of mitigation strategies for road traffic in a sufficiently accurate manner. As TransClim simulates about 6000 times faster than a complex chemistry climate model, it enables to quantify the effect of many emission scenarios in different regions.

Aviation activities contribute substantially to the anthropogenic climate impact. Due to an increasing demand on aviation transport, multiple mitigation strategies have been established to reduce the contribution to climate change by aviation. One promising strategy is to re-route aircraft, such that climate sensitive atmospheric areas are avoided. This mitigation strategy, depends on the scientific understanding of all processes involved. The European project REACT4C assessed the feasibility of such a mitigation technique by simulating the climate impact of NOx, as well as other emissions and contrail formation for eight distinct weather pattern. For each weather pattern, unit emissions of NOx are emitted in the North Atlantic flight sector. Each air parcel, containing the emitted NOx, is tracked within the atmosphere. This unique model set-up allows to analyse concentration changes of O3 and CH4 along each trajectory. In general, due to the emitted NOx, O3 is produced and CH4 is lost. Most recent results showed that by just increasing the operation cost by 1%, the climate impact can be reduced by about 10%. By comparing climate cost functions (CCF), a metric of the climate impact per unit emission, to weather charts, a link between high pressure ridges and the total climate impact of NOx is observed. Therefore, this research focuses on identifying weather influences on the temporal development of O3 and CH4 due to aviation attributed NOx emission.

In this thesis, the NOx chemistry, atmospheric transport processes and the model set-up of the REACT4C project is reviewed. The temporal development analysis of O3 is split-up into two parts, the O3 build-up and the O3 depletion. First, all data from the climate model are re-gridded and chemical production and loss rates are isolated from all other loss terms (i.e. diffusion). Certain characteristics of the temporal concentration changes of O3 are identified. A systematic analysis of the background chemical compounds and all important chemical reactions involved, provide insides to identify seasonal and emission altitude differences. With the help from literature and multiple statistical means, weather influences on those production and loss terms and thus the temporal development of O3 and CH4 , are identified. In a final step, inter-seasonal variations are analysed.

In general, the chemical processes during the O3 build-up are dominated by the emitted NOx, whereas the chemical processes during the depletion of O3 are dominated by the high O3 concentration. Seasonal differences of the maximum O3 concentration and the total CH4 loss are caused by lower background concentrations of all chemicals involved during winter, which lead to lower production and loss rates of O3 and CH4 . At the same time altitude differences in the production and loss of O3 and CH4 are caused by altitude variations in all chemicals involved.

The vertical transport within the atmosphere defines the time when the O3 maximum is reached. If an air parcel containing the emitted NOx, is transported fast to a lower altitude, the O3 maximum occurs sooner. If however the same air parcel would stay for a longer time at a high altitude, a late O3 maximum occurs. It could be identified that this downward motion is caused by the subsidence within a high pressure system. Airparcel with an earlier O3 maxima, experience high subsidence, which leads to a higher chemical activity based on higher temperatures. During summer a high O3 maximum can only be reached, if the background concentration of NOx is low during the O3 build-up. If the background NOx concentration is high, only very low O3 maxima occur. During winter the maximum O3 concentration is limited by the background concentration of HO2 . Only high HO2 background concentrations lead to high O3 maxima. The temporal development of CH4 is mainly influenced by the maximum O3 concentration as well as specific humidity. High O3 and H2O concentrations lead to high OH productions, which lead to a high CH4 losses. A high CH4 loss only occurs, if the maximum O3 concentrations and the specific humidity are high.

This study shows that the weather situation each air parcel, containing NOx emissions, experiences has a direct influence on the resulting concentration changes of O3 and CH4 . Therefore, weather has a direct impact on the climate impact of NOx , since the concentration change of O3 and CH4 directly influences the resulting climate impact. The understanding of processes related to the climate impact of aviation attributed NOx emission is increased. This improved understanding shows great potential to improve possibilities to forecast local climate impact resulting from aviation NOx emissions, which is necessary for future re-routing mitigation strategies.